We've discovered 500 exoplanets, most of them gas giants even bigger than Jupiter. But some, known as super-Earths, are rocky worlds only a bit larger than our planet, and now we've discovered the cloudy atmosphere of one of these worlds.

GJ 1214b is about 6.5 times Earth's mass and located 40 light-years away. When it was first discovered in 2009, astronomers measured its relatively low density, which indicates it has an atmosphere, but they had been unable to measure it.

Harvard astrophysicist Jacob Bean led a new line of research that examined light from GJ 1214b's star. Specifically, they were looking for starlight from when GJ 1214b was between the star and Earth, meaning it had passed through the planet's atmosphere. Depending on the chemical composition of the atmosphere, the light would be subtly altered, creating gaps in the wavelength that would reveal the gases with which it interacted.

The spectrum they got back from GJ 1214b was featureless, meaning the light was unaffected by transit through the planet. That may sound like a failure, but it actually rules out some prime suspects, including a cloud-free, hydrogen-rich atmosphere like that of Neptune. Hydrogen, as the lightest element, is the most prone to absorbing passing sunlight as it's the least tethered to its planet. Bean explains:

"A hydrogen-dominated atmosphere would be very 'puffy.' It is this puffiness that would have given a very strong signature in the spectrum that we measured. The featureless spectrum tells us that it is probably a very dense atmosphere. However, the alternative is that it does have a puffy atmosphere but with thick, high clouds that we can't see through, similar to Venus, or [Saturn's largest Moon] Titan."

If that's true, then the planet is most likely shrouded in water vapor. Because GJ 1214b is very close to its star, the entire planet might well look to be covered in steam. Bean says we should know more for certain very soon:

"I think we'll get the answer within a year, maybe even sooner, we just need longer wavelength observations. Whilst clouds and hazes give a uniform absorption over the wavelength range we used, over very large wavelengths you would expect a difference."

Still, there's some room for caution. As Open University exoplanet researcher Carole Haswell explains, we still need a little more confirmation of the findings from a space-based observer:

"They've done this looking through the Earth's atmosphere, which is never a friend to astronomy," Carole Haswell, an exoplanet researcher at the Open University, told physicsworld.com. "What they've done is very difficult; any slight systematic effects are going to have a huge effect on the conclusions that you draw. It's good, solid and exciting stuff but I'd like to see it checked from space, e.g. with Hubble."

She is, however, excited about what this could mean for the search for Earth-like planets:

"If you can measure the composition of the atmospheres of planets like GJ 1214b then you are getting quite close to saying how similar they are to Earth. This is a big step in addressing the question of whether Earth is unique."